1. Field of the Invention
The present invention relates to a water-decomposable fibrous sheet capable of being readily decomposed and dispersed in water flow. More precisely, it relates to such a water-decomposable fibrous sheet resistant to surface friction.
2. Description of the Related Art
To wipe the skin of human bodies including the private parts thereof, or to clean toilets and thereabouts, used are disposable cleaning sheets made of paper or non-woven fabric. For these cleaning sheets, water-decomposable cleaning sheets that could be directly disposed of in toilets after use have been developed, as being convenient for such purposes. The degree of their decomposability in water must be high in some degree. This is because, if poorly water-decomposable cleaning sheets are disposed of in toilets after use, they will take a lot of time until they are decomposed and dispersed in septic tanks, or will clog the drainpipes around toilets, etc.
For wiping off wet dirt and for easy and effective use, many cleaning sheets for wiper applications are packaged while being wetted with a liquid detergent chemical or the like, and are put on the market. Therefore, such water-decomposable cleaning sheets must have high strength in wet to such a degree that they are well fit for wiping with them wetted with such a liquid chemical or the like, but must well decompose in water after they are disposed of in toilets.
For example, Japanese Patent Publication No. 24636/1995 discloses a water-decomposable cleaning article that comprises a water-soluble binder having a carboxyl group, a metal ion and an organic solvent. However, the metal ion and the organic solvent irritate the skin.
Japanese Patent Laid-open No. 292924/1991 discloses a water-decomposable cleaning article of polyvinyl alcohol-containing fibers with an aqueous solution of boric acid infiltrated thereinto; and Japanese Patent Laid-Open No. 198778/1994 discloses a water-decomposable napkin of polyvinyl alcohol-containing non-woven fabric with a borate ion and a bicarbonate ion introduced thereinto. However, polyvinyl alcohol is not resistant to heat, and therefore the wet strength of the water-decomposable cleaning article and the water-decomdosable napkin is lowered at 40xc2x0 C. or higher.
Recently, various water-decomposable absorbent articles including sanitary napkins, panty liners, disposable diapers and others have been investigated in the art. In view of their safety, however, the water-decomposable fibrous sheets mentioned above could not be used as the top sheets for those absorbent articles that shall be kept in direct contact with the skin for a long period of time, as they contain a binder and an electrolyte.
On the other hand, Japanese Patent Laid-Open No. 228214/1997 discloses a water-degradable non-woven fabric having a wet strength of from 100 to 800 gf/25 mm (from 0.98 to 7.84 N/25 mm) as measured according to JIS P-8135, which is produced by mixing fibers having a length of from 4 to 20 mm with pulp followed by entangling them through treatment with high-pressure water jets. Since the constituent fibers are entangled in it, the non-woven fabric disclosed has a bulky feel. However, in producing the non-woven fabric, long fibers are entangled through high-pressure water jet treatment, whereby the non-woven fabric produced could have such a relatively high wet strength. Therefore, according to the technique disclosed, it is difficult to realize well-balanced bulkiness, strength and water-degradability for the non-woven fabric produced, and the non-woven fabric produced is unsuitable to disposal in flush toilets, etc.
An object of the present invention is to provide a water-decomposable fibrous sheet which is well decomposed in water and has good strength enough for practical use even though no binder is added thereto.
Specifically, the invention is to provide a water-decomposable fibrous sheet comprising fibers containing at least 3% by mass of fibrillated rayon, the fibrillated rayon having a degree of beating of at most 700 cc and having primary fibers of a predetermined fiber length and microfibers extending from the primary fibers;
wherein the microfibers are entangled with at least either of other microfibers and other fibers therein, and
the surface friction resistance of the fibrous sheet in dry, measured according to the abrasion resistance test method of JIS P-8136, is at least three rubbing cycles.
Naturally in dry and even in wet with water, the water-decomposable fibrous sheet of the invention all the time keeps high strength. When it is immersed in a large amount of water after used and disposed of in toilets and others, it is readily decomposed. In the fibrous sheet of the invention, the microfibers of fibrillated rayon are entangled with and are further hydrogen-bonded to other fibers and other microfibers therein, thereby exhibiting their ability to bond fibers constituting the sheet and to enhance the strength of the sheet. When the fibrous sheet receives a large amount of water applied thereto, the entangled microfibers therein are loosened or the hydrogen bonds between the bonded microfibers therein are broken, whereby the fibrous sheet is readily decomposed in water.
In addition, the surface of the water-decomposable fibrous sheet of the invention is highly resistant to friction. The sheet surface contains many microfibers, and the microfibers therein are principally brought into direct contact with the surfaces of other objects. Accordingly, during use, the overall friction that the fibrous sheet will directly receive will be reduced, and the sheet surface will be hardly broken even when rubbed against other objects and could keep a predetermined strength. Therefore, when the fibrous sheet is used as a wiper sheet or as a top sheet for absorbent articles, it is not broken and gives a comfortable feel to users.
The water-decomposable fibrous sheet of the invention can be composed of materials not harmful to human bodies.
Preferably, the surface friction resistance of the fibrous sheet in wet is at least three rubbing cycles.
Also preferably, the sheet surface is pressed under heat so that the microfibers of the fibrillated rayon therein are hydrogen-bonded to at least either of other microfibers and other fibers therein.
Also preferably, the fibrillated rayon in the fibrous sheet is such that the length of the primary fibers constituting it falls between 1.8 mm and 10 mm at the peak of its self-weighted, average fiber length distribution profile curve, and that the microfibers having a length of at most 1 mm account for from 0.1 to 65% by mass of the self-weight of the fibrillated rayon.
Also preferably, the water-decomposable fibrous sheet has a multi-layered structure containing the fibrillated rayon in at least one of two surface layers.
The fibrous sheet may be a non-woven fabric processed through water-jetting treatment, or it may be produced in a paper-making process.
Preferably, the degree of fineness of the fibrillated rayon falls between 1.1 and 1.9 dtex.
Also preferably, the weight of the fibers (this may be referred to as xe2x80x9cMetsukexe2x80x9d) of the fibrous sheet falls between 20 and 100 g/m2.
Also preferably, the decomposability in water of the fibrous sheet, measured according to JIS P-4501, is at most 200 seconds.
Also preferably, the wet strength of the fibrous sheet is at least 1.1 N/25 mm.
Also preferably, the dry strength of the fibrous sheet is at least 3.4 N/25 mm.
The water-decomposable fibrous sheet of the invention can be produced according to a method that comprises;
(A) a step of sheeting fibers into a fibrous web, in which the fibers contain fibrillated rayon that comprises primary fibers having a predetermined fiber length and microfibers extending from the primary fibers and has a degree of beating of at most 700 cc, and
(B) a step of pressing the fibrous web under heat while the surface of the fibrous web is wetted with water, whereby the microfibers existing in the surface are hydrogen-bonded to at least either of other microfibers and other fibers therein.
The production method may include a step (C) of processing the fibrous web through water-jetting treatment between the step (A) and the step (B).